Washington said Monday it might use a critical fifth round of nuclear talks between Iran and world powers to discuss with Tehran possible cooperation on tackling the crisis raging in Iraq.

The United States and Iran, which have been bitter foes for more than 30 years, are both deeply concerned by a major insurgency by Sunni militants who have overrun swathes of Iraq over the past week.

A senior US official said that as a result “there may be some conversations” with Iranian negotiators on the sidelines of nuclear talks between Iran and six world powers in Vienna that began on Monday.